Conventionally, holograms have mainly been used in the fields of stereoscopic image display and security. As materials for the holograms, wet-type materials that require development such as silver salt photosensitive materials and dichromate gelatin, have been mainly used in old times. Recently, however, a dry-type photopolymerizable hologram recording material excellent in environment resistance and light resistance after recording, which does not require any development process, has become the mainstream. A recording mechanism of the photopolymerizable hologram recording material, which is different from that of the silver salt photosensitive material, the dichromate gelatin, and the like, is generally considered as follows. That is, when an interference pattern composed of the interference of light excellent in coherence is irradiated onto a recording material, substances having different polymerization abilities respectively diffuse and migrate to a bright part of the light (higher light intensity area) and a dark part of the light (lower light intensity area) to form a refractive-index modulated structure as the photopolymerization proceeds, thereby recording a hologram. There are three types of polymerization forms of photopolymerizable hologram recording materials: (1) a radical polymerization type; (2) a cationic polymerization type; and (3) a radical/cationic polymerization combination type. The photoradical polymerization-type materials are commonly used. However, there is a disadvantage in that even though the sensibility is high, shrinkage attributable to the polymerization form is as high as several percents. Currently, among the photopolymerizable hologram recording materials, OMNIDEX manufactured by DuPont is the only photosensitive resin composition available in the market.
In recent years, for the realization of a ubiquitous information society, studies aiming at using photopolymerizable hologram recording materials for holographic data storages have become active again all over the world, and various photopolymerizable hologram recording materials have been proposed. Drastic progressions in technologies of laser optical sources, spatial light modulators, and so on while promoting national projects Holographic Data Storage System (HDSS, 1955-1999) and Photorefractive Information Storage Materials (PRISM, 1994-1998) that have started in the United States from the mid 1990s are contributable to such movements. A conventional technology for recording data on an optical disk such as a CD or a DVD is carried out such that laser beams are converged on a recording material through a lens to record data thereon bit by bit. The data-recording capacity defined by a size of a beam-converged spot has already come to its theoretical limit, so a new technology has been expected for a further increase in recording capacity. One of the technologies that has been attracting attention in recent years is holography.
The holographic data storage can perform volume recording instead of in-phase recording such as in CDs and DVDs, and data to be handled is page data, so the holographic data storage can record a large amount of data in comparable to the conventional CDs and DVDs. Currently, in each research institution, material development and recording technology development are actively performed toward putting the Write Once Read Many (WORM) type hologram optical disc capable of performing 1 terabyte (TB) recording to practical use. According to a basic design concept of a material for the holographic data storage, the material to be used is the same as the photopolymerizable hologram recording material for use in stereoscopic image recording or security as mentioned above. However, for use in the photographic data storage, requirements for the material, particularly sensitivity to an optical source to be used and cure shrinkage, are extremely strict. Typical examples of the photosensitive resin composition for the holographic data storage include one available from DCE Aprilis, Inc. in the United States affiliated with Polaroid Corporation in the United States and one available from InPhase Technologies Inc. in the United States affiliated with Lucent Technologies.
First, for example, as described in Patent Documents 1 and 2, DCE Aprilis, Inc. in the United States discloses materials with controlled polymerization shrinkage prepared by using Cationic Ring-Opening Polymerization (CROP) monomers having oxirane and oxetane rings and obtained through pre-photoirradiation. However, although cationic polymerization shows a small degree of enzyme inhibition as in radical polymerization, the sensitivity thereof is slightly deteriorated in a long-wavelength region. In contrast, for example, as described in Patent Document 3, 4 or 5, InPhase Technologies Inc. in the United States discloses a material having an extremely small polymerization shrinkage of 1% or less, which is attained by using cyclohexane oxide in combination with an expanding agent (diphenylfuran carboxylate). Also in this case, however, cure shrinkage is prevented by whole-area exposure before or after recording using a cationic ring-opening polymerizable monomer. However, no proposal has been made to improve the sensitivity. For putting the holographic data storage to practical use, a further improvement in photosensitivity, a reduction in cure shrinkage, and an improvement in thermostability are expected.
Patent Document 1: U.S. Pat. No. 5,759,721                Patent Document 2: U.S. Pat. No. 6,784,300        Patent Document 3: U.S. Pat. No. 3,993,485        Patent Document 4: U.S. Pat. No. 6,124,076        Patent Document 5: JP 2000-086914 A        Patent Document 6: JP 5-94014 A        Patent Document 7: JP 9-106242 A        Patent Document 8: JP 2004-123873 A        
In Patent Documents 6 and 7, a hologram photosensitive resin composition containing as a blended mixture of radically polymerizable ethylene monomers, a photopolymerization initiator, and an epoxy resin is described. In this composition, the radically polymerizable ethylene monomers are preferentially polymerized from the bright portion of an interference pattern at the time of hologram exposure with laser. A hologram is formed by using the phenomenon in which the ethylene monomers migrate to the bright portion. Note that the epoxy resin is cured after that. Patent Document 8 describes a soluble aromatic copolymer aromatic copolymer having a structural unit of a divinyl aromatic compound and a structural unit of a monovinyl aromatic compound, but does not disclose the use of the hologram.